Bond behavior of the interface between concrete and basalt fiber reinforced polymer bar after freeze–thaw cycles

Abstract

The shear bond of interface between concrete and basalt fiber reinforced polymer (BFRP) bars during freeze–thaw (F–T) cycles is crucial for the application of BFRP bar-reinforced concrete structures in cold regions. In this study, 48 groups of pull-out specimens were designed to test the shear bond of the BFRP-concrete interface subjected to F–T cycles. The effects of concrete strength, diameter, and embedment length of BFRP rebar were investigated under numerous F–T cycles. Test results showed that a larger diameter or longer embedment length of BFRP rebar resulted in lower interfacial shear bond behavior, such as interfacial bond strength, initial stiffness, and energy absorption, after the interface goes through F–T cycles. However, higher concrete strength and fewer F–T cycles were beneficial for enhancing the interfacial bond behavior. Subsequently, a three-dimensional (3D) interfacial model based on the finite element method was developed, and the interfacial bond behavior of the specimens was analyzed in-depth. Finally, a degradation bond strength subjected to F–T cycles was predicted by a proposed mechanical model. The predictions were fully consistent with the tested results. The model demonstrated accuracy in describing the shear bond behavior of the interface under numerous F–T cycles.

Graphical abstract

Keywords

F–T cycle / interface / shear bond strength / bond stress−slip curves / bond degradation

1 Introduction

Corrosion of steel bars in reinforced concrete structures significantly degrades their performance [1]. The basalt fiber reinforced polymer (BFRP) rebar is a more recent type of structural reinforcement characterized by excellent strength, good corrosion resistance, and lighter weight. Furthermore, BFRP costs less than glass or carbon fibers [1–3].

In recent years, numerous studies have been implemented to explore the mechanism and durability of structures reinforced by BFRP rebars. For instance, Attia et al. [4] demonstrated that basalt fiber reinforced concrete-BFRP slabs exhibited larger strains and ultimate capacities than plain specimens. Duic et al. [5] observed that the BFRP-reinforced concrete beams showed good deformability in tests. Additionally, they determined that BFRP stirrups were beneficial in delaying the shear failure for beams with a low reinforcement ratio. This finding was also supported by results reported by Abushanab et al. [6]. Duic et al. [5] further revealed that the interfacial bond mechanism was critical for BFRP reinforced concrete structures.

The bond behavior of the BFRP bar-concrete interface was a critical aspect of structural performance, as demonstrated in several studies [7,8]. The bond behavior was found to rely on the initial chemical adhesion, the mechanical interlock, and the friction between concrete and BFRP bars [9]. Refai et al. [10] tested 36 concrete-BFRP bars interfacial specimens and revealed that the interfacial bond-slip curves could be described in analytical models.

Hassan et al. [11] explored the influence of the exposure duration and temperatures on the interfacial bond of the concrete-BFRP interface in an alkaline environment. They predicted that the bond strength would remain between 71% and 92% after 50 years in alkaline environments with average annual temperatures ranging from 5 to 35 °C. Wei et al. [12] found that the interfacial bond strength significantly decreased after a dry–wet cycle. Furthermore, Shen et al. [13] quantified the interfacial bond behavior under cyclic loads using the proposed analytical model. Ceroni et al. [14] investigated the bond properties between CFRP and concrete in several environments using single push-pull shear and beam tests. They found that the bond degradation was higher as the environmental temperature increased. Additionally, similar tests were performed by de Domenico et al. [15]. They found that the bond strength between CFRP and concrete decreased more than 30% in passing from the non-conditioned to conditioned class at 50 °C, but no reduction was observed at 30 °C.

Sun et al. [16] revealed that using recycled aggregate concrete degraded the interfacial bond behavior. Meanwhile, Wang et al. [17] built a CMR model to account for the ascent stage in the bond stress−slip curves between BFRP bars and recycled aggregate concrete. Previous studies on interfacial bond properties between concrete and BFRP bars in various environments have been a crucial advancement. However, the most pressing challenge in using BFRP bars or any type of fiber reinforced polymer (FRP) is the limited bonding performance of interfacial bonds under freeze–thaw (F–T) cycles. In cold regions, the degradation of the interfacial bond during F–T cycles has to be considered; otherwise, the capacity and integrity of BFRP bar-reinforced concrete structures may be overestimated, which can cause safety problems [18].

In this study, the bond behavior of the concrete-BFRP bar interface after F–T cycles was explored using pull-out experiments, and the results are presented in Sections 2 and 3. Moreover, Section 3 shows a 3D model of the interfacial bond behavior between BFRP and concrete developed based on finite element analysis (FEA) to analyze the interfacial behavior during and after F–T cycles. Finally, by combining the experimental and numerical results, a mechanical model was proposed to predict the degradation of the interfacial bond behavior under F–T cycles, which is discussed in Section 4. Significant results are discussed in Section 5.

2 Experiment program

2.1 Preparation of concrete matrix and basalt fiber reinforced polymer bars

To investigate the effects of concrete strength on interfacial bond properties after F–T cycles, three different grades of concrete mixtures (C40, C50, and C60) were prepared, with their mixture proportions listed in Tab.1. The tests utilized Bohai Brand Portland cement (P.O. 52.5) and natural middle sands as fine aggregate, with a bulk density of 2630 kg/m³ and a fineness modulus of 2.6. The coarse aggregate was stone crushed with particle sizes ranging from 5 to 20 mm, with an average content of 44.0%, 49.9%, and 6.1% for coarse aggregates between 5–10, 10–16, and 16–20 mm, respectively. Moreover, a polycarboxylic type of superplasticizer (SP) with a water reduction capacity of 20% was used in the experiment to improve the working properties of the fresh concrete [17].

Twenty-four concrete cubes, each with a side of 100 mm for each grade, were prepared for mechanical strength testing. The fresh concrete was poured into molds and left to cure for 24 h before demolding. As shown in Tab.2, six cubic concrete specimens for each grade (18 specimens in total) were cured at 25 °C and a relative humidity (RH) of 95% for 28 d. The remaining 18 specimens for each grade (54 specimens in total) were subjected to F–T cycles, as discussed in detail in Subsection 2.2, after being maintained for 24 d.

To investigate the effects of BFRP diameter on its shear bond with concrete, three types of BFRP bars, R1, R2, and R3, with diameters of 12, 10, and 8 mm, respectively, were provided by Zhejiang Shijin Basalt Fiber Company Limited. Additionally, the tensile strength of R1, R2 and R3 are 729, 722, and 688 MPa, respectively. The elasticity modulus of R1, R2, and R3 are 24.4, 24.5, and 24.2 GPa, respectively. Furthermore, BFRP bars of R2 were selected to undergo multiple F–T cycles to study the degradation of their mechanical properties.

2.2 Preparation of interfacial bond specimens

In this study, pull-out tests were conducted to determine the interfacial shear bond properties between concrete and BFRP bars. Based on the standard of GB/T 30022 [19] and ACI 440 [20], 16 groups of 48 pull-out specimens were prepared, as shown in Fig.1. The concrete matrix was 100 mm × 100 mm × 100 mm in size, with a BFRP bar positioned through its central axis. To consider the effects of the BFRP bar embedment length, Poly Vinyl Chloride (PVC) pipes with an inner diameter of 4 mm larger than that of the BFRP bar were prepared for the BFRP bar-concrete F–T tests. Each specimen was prepared as follows. 1) The BFRP bar was pushed through a pre-set PVC pipe along the central axis, and softened paraffin was used to fill the gaps between the bar and the PVC pipe. The paraffin also served as a barrier to prevent the fresh concrete matrix from flowing into the pipe. 2) The bar was positioned inside the PVC pipe as it passed through the pre-set holes along the central axial, and a 50-mm BFRP bar was prepared at another free end to connect to a linear variable differential transformer (LVDT). 3) A fresh concrete mixture was cast into a form, and then removed after 24 h. The specimens in the control group were cured for 28 d, while the other test specimens were maintained for 24 d before being subjected to 100, 200, and 300 F–T cycles in the laboratory.

The effects on these interfacial specimens were used to quantify the concrete strength, diameter of BFRP bars, and embedment length after each F–T test, in that order. Additionally, the interfacial bond properties were analyzed.

2.3 Procedure of the freezing and thawing tests

The F–T cycling test used the “slow freeze method” according to GB/T50082-2009 [21]. Initially, all specimens were removed from the curing room and submerged in water at (20 ± 2) °C for 4 d. Subsequently, the F–T cycling test was performed as follows. (1) All the specimens were frozen for 6 h in a freezer, with the minimum center temperature of the specimens maintained at (−20 ± 2) °C. (2) All the specimens were thawed by immersion in water for 2 h at (20 ± 2) °C. To evaluate the effects of the F–T cycles on the shear bond properties, the specimens in each group were subjected to 100, 200, and 300 F–T cycles.

2.4 Test procedure

The interfacial shear bond performance was characterized by the device shown in Fig.2. In this apparatus, a clamp was used to secure steel bar to the top steel plate of the frame, which was affixed to the top of the Unitoron Universal Tester. At the bottom of the tester, the steel pipe was fastened to the tester for loading. Moreover, three linear displacement gauges were fixed to the bottom and top of the BFRP bar to measure the relative slip of the interface between the BFRP bar and concrete. All tests were performed using a constant force rate of 0.05 kN/s. All experimental data were collected automatically using the data acquisition equipment in the civil engineering laboratory of the Hefei University of Technology.

3 Experimental results and analysis

3.1 Performance of the concrete matrix subjected to freeze–thaw cycles

The weight of concrete specimens was recorded after varying periods of F–T cycles, and it is shown in Fig.3, where the value represents the average result of the three tested specimens. No significant reduction in concrete weight was observed, which is consistent with the findings of Shang et al. [22]. Additionally, the desquamation and cracks on the concrete surface were apparent after the F–T cycle, as shown in Fig.4(a).

Moreover, the strength of concrete specimens subjected to uniaxial compression and splitting tension after F–T cycles of 0, 100, 200, and 300 times are presented in Fig.5(a) and Fig.5(b), respectively. Both compressive strength and splitting tensile strength decreased as the number of F–T cycles increased. For example, the compressive strength of C40 decreased by 38.5%, 25.9%, and 9.4%, and the corresponding splitting tensile strength of concrete decreased by 31.0%, 24.5%, and 9.1% after 300, 200, and 100 F–T cycles, respectively. This is consistent with the findings of Wang et al. [23], who reported a reduction of 28.1% and 27.9% in compressive strength and splitting tensile strength of concrete, respectively, after 100 F–T cycles. The gradual loss of strength is attributed to the production of more cracks in internal structures due to an increased number of F–T cycles (at 100 cycles per session) [24]. The failure modes of the concrete cubes subjected to different F–T cycle sessions (200 cycles each) under compression are shown in Fig.4(b).

Additionally, the degradation of concrete strength was more apparent for lower-grade concrete. Particularly, after 300 F–T cycles, the compressive strength of C40, C50, and C60 concrete decreased by 38.5%, 30.1%, and 17.8%, and the splitting tensile strength decreased by 31.0%, 29.6%, and 13.0%, respectively. This is attributed to the higher water/cement ratio, which leads to more cracks and increases the damage probability in the concrete matrix [24]. Moreover, the failure modes of the concrete splitting tensile specimens subjected to different F–T cycle durations are presented in Fig.4(c).

3.2 Performance of basalt fiber reinforced polymer bars after freeze–thaw cycles

The elastic modulus, tensile strength, and maximum tensile strain for BFRP bars were tested for numerous F–T cycles, and the results are listed in Tab.3. Each value is the average of three specimens tested in each set. Overall, both the tensile strength and maximum tensile strain for BFRP bars decreased steadily with an increasing number of F–T cycles. Particularly, compared with the bar without F–T cycles, the tensile strength of BFRP bar decreased by 25.7%, 42.2%, and 53.2%, and the maximum tensile strain of bar decreased by 25.0%, 41.3%, and 43.0% after being subjected to 100, 200, and 300 F–T cycles, respectively. Shi et al. also found experimentally that the F–T cycle had a negligible effect on the tensile properties of BFRP sheets, but the decrement was not significant [25]. Moreover, the elastic modulus of the BFRP bar showed a slight increment, that is, 5% and 7%, after being subjected to 200 and 300 of F–T cycles, indicating that F–T cycles can lead to more brittle BFRP bars, which agrees with the findings of Shi et al. [25]. They obtained an increment of 10% and 13% for the BFRP sheet after being subjected to 100 and 200 F–T cycles.

3.3 Shear bond behavior of the interface

3.3.1 Failure modes

In our study, failure was classified as interfacial bond failure, concrete tensile failure, and BFRP bar rupture [26]. However, only the interfacial pull-out failure and concrete tensile failure were found in our experiment, and no BFRP bar rupture occurred. This can be attributed to the high elastic modulus and strong tensile strength of the BFRP bar. The pull-out failure is shown in Fig.6(a), where BFRP bars were pulled out from the concrete matrix. No obvious erosions were found in the concrete. This phenomenon can be attributed to the lower interlock between the concrete matrix and BFRP bar, compared with the strength of the concrete matrix and deformed bar [27]. Conversely, when the interlock between the concrete and BFRP bar was high enough, splitting failure occurred, and obvious cracks appeared around the concrete matrix, as shown in Fig.6(b).

3.3.2 Bonding stress−slip curves of interface

To simplify, the interfacial shear bond stress in the pull-out experiment can be defined as follows (Eq. (1)) [28]:

(1){\tau }_{\text{i}} =\frac{P}{\text{π}\cdot d\cdot l_{\mathrm{e}}},

where τ_i is the shear bond stress of the concrete-BFRP bar interface, P is defined as the pull-out load, while d and l_e represent the BFRP diameter and embedment length, respectively.

Fig.7(a)–Fig.7(g) plot the strength effects of the concrete matrix, BFRP diameter, BFRP embedment length, and F–T cycles on the interfacial shear bond stress−slip relationship. All curves exhibit ascending and descending segments. Initially, the interfacial shear bond stress increased nonlinearly up to the ultimate value, which corresponds to the ascent stage. As the load increases, a sudden drop is observed once the concrete matrix cracks, as shown in the descent stage. This drop occurs because the interfacial bond strength exceeds the critical friction between the concrete matrix and BFRP bar [27].

The interfacial bond stress−slip curves are shown in Fig.7(a) and Fig.7(b) for different strengths of concrete matrix. Notably, specimens with higher strength concrete generally exhibit lower bond slip and higher shear bond stress, such as D0-C60-d10-50 and D200-C60-d10-50, regardless of F–T cycles, indicating that these specimens were capable of absorbing more energy during pull-out. This behavior can be attributed to the larger interfacial adhesion provided by the higher strength of the concrete matrix.

However, as seen in Fig.7(c)–Fig.7(f), both larger BFRP diameter and longer embedment length result in lower interfacial stress and a smaller slip, indicating that they consumed less energy during pull-out tests, regardless of whether the specimen has undergone F–T. This behavior can be attributed to the inverse relation between bond stress and BFRP diameter or embedment length, as described by Eq. (1).

Finally, Fig.7(g) describes the effect of F–T cycle frequency on the interfacial shear bond stress−slip curves. Increasing F–T cycles resulted in weaker shear bond, and fewer pull-out slips were recorded, indicating that the specimens absorbed less energy during the pull-out tests. In conclusion, higher strength concrete is beneficial for enhancing the interfacial bond properties between concrete and BFRP bar, while neither BFRP bar diameter nor embedment length nor F–T cycles promote interfacial bond behavior.

3.3.3 Interfacial shear bond strength

The interfacial shear bond strength for each pull-out specimen can be calculated using Eq. (1) when the applied load, P, reaches its maximum pressure. Fig.8(a) to Fig.8(d) reveal the effects of concrete strength, BFRP diameter, embedment length, and F–T cycles on the interfacial shear bond strength.

Fig.8(a) shows that the interface expressed a stronger shear bond strength with a higher strength of the concrete matrix, regardless of F–T cycles. Particularly, the shear bond strength increased by 163.9% as the concrete compressive strength increased from 43.9 to 65.9 MPa without F–T cycles. After 200 F–T cycles, the shear bond strength with C60 concrete exhibits a 103.3% higher increase from 6.11 to 12.42 MPa. Fig.8(a) shows that the bond strength of the interface subjected to F–T cycles increases slowly compared to those without F–T cycles. This is similar to the findings of Deng et al. [29], who observed that concrete strength had a beneficial effect on the interfacial bond strength between BFRP bars and lightweight aggregate concrete under F–T cycles. This effect is due to the performance degradation of the concrete matrix after numerous F–T cycles.

Additionally, the effects of BFRP embedment on the interfacial shear bond strength are illustrated in Fig.8(b), where the interfacial bond strength with a 50-mm and 60-mm BFRP bar embedments decreased by 20.7% and 31.9%, respectively, compared to a 40-mm embedment. Thus, the longer BFRP embedment resulted in a weaker bond between the concrete matrix and BFRP bar. This effect can be attributed to the longer bar embedment length, requiring more concrete to achieve the equivalent normal bond stress, resulting in lower interfacial bond strength.

Furthermore, Fig.8(c) shows that the interfacial bond strength decreased substantially when the BFRP diameter increased, which is consistent with previous research experimental results [27,30]. For example, the interfacial specimens with 10-mm and 12-mm-diameter BFRP bars lost 20.7% and 31.9% of shear bond strength, respectively, compared with an 8-mm-diameter BFRP bar without F–T cycles. However, the interfacial shear bond strength decreased by 40.1% and 50.6%, after 200 F–T cycles. This tendency can be attributed to the performance degradation of the concrete matrix and BFRP bars after numerous F–T cycles.

Finally, Fig.8(d) shows that the interfacial shear bond dropped sharply after the pull-out specimens were subjected to F–T cycles. Particularly, the interfacial shear bond decreased by 24%, 43.6%, and 52.5% when the number of F–T cycles increased from 0 to 100, 200, and 300, respectively. These results are consistent with those reported in previous studies [23,29]. For instance, Wang et al. [23] found that the interfacial shear bond strength between BFRP and recycled aggregate thermal insulation concrete was reduced by 19.8% and 38.7% after being subjected to 50 and 100 F–T cycles, respectively. The strength of the concrete matrix and BFRP bar decreased with an increasing number of F–T cycles, leading to a lower adhesion and friction between the concrete and BFRP bar.

4 Numerical analysis of interfacial specimens under freeze–thaw cycles

4.1 Numerical method

To investigate the interfacial bond degradation between the concrete matrix and BFRP bar after F–T cycles, a series of 3D numerical models of the tested specimens were constructed using Abaqus finite element code (version 2020).

4.1.1 Element types of concrete matrix and basalt fiber reinforced polymer bars

The concrete matrix was modeled as an 8-node 3D solid homogeneous element, referred to as C3D8R, which can illustrate both tensile cracking and compressive crushing of material. Moreover, the concrete damaged plasticity (CDP) model was applied to illustrate the tensile and compressive failure and permanent strains of concrete. To consider the effect of F–T cycles on the concrete properties, the damage principal relationship of concrete subjects to F–T cycles proposed by Long et al. [31] was adopted and can be expressed as follows:

(2){\sigma }_{\mathrm{n}}=(1- D_{\mathrm{n}})E_0\epsilon ,

(3)D_{\mathrm{n}}=1- \frac{E_{\mathrm{n}}}{E_0},

where {\sigma }_{\mathrm{n}} is the concrete stress after n F–T cycles, D_{\mathrm{n}} is the damage variable of concrete after n F–T cycle, ε is the concrete strain, and E_n and E₀ are the elastic modular of concrete subjected to n and zero F–T cycles, respectively.

Furthermore, the C3D8R element was applied to simulate the BFRP bar in the interfacial specimen as a linear elastic model. The mechanical properties of the concrete matrix and BFRP bars were determined from the test results, as shown in Fig.5 and Tab.4, respectively.

4.1.2 Element types of concrete-basalt fiber reinforced polymer bar interface

A cohesive element with zero thickness was embedded between the concrete and BFRP bar to simulate the interfacial behavior. The exponential traction-separation criterion was adopted to simulate the bonding and debonding between the concrete and BFRP bar due to its excellent ability of convergence and emulation [32]. The criteria are given as follows:

(4)

(5)

(6)D=\frac{\delta -{\delta }_0}{{\delta }_{\mathrm{c}}-{\delta }_0} ,

where T_i and δ are the pull-out load and corresponding relative displacement of the interfacial specimen, δ₀ and δ_c are the relative displacements of the BFRP bar sliding after starting at δ₀ and completing at δ_c, k₀, and k_c are the interfacial stiffness before and after damage initiation, respectively, which are equal to the slope of the rising and falling curves. These parameters were recorded during our experiments. D is the scalar degradation stiffness, which was calculated using Eq. (6), D is equivalent to 1 when δ = δ_c, and D is equivalent to 0 when δ = δ₀. The mechanical properties of the interface used in the simulation were obtained from our experiments and are listed in Tab.4.

In the simulation, the displacement as a loading condition was applied at the top surface of the BFRP bar along the z-axis, while the movement of the concrete matrix was restricted as a fixed end. The numerical specimen was meshed with 5 mm-sized elements, chosen for their ability to produce high-quality results with high efficiency and accuracy, as indicated by multiple calculations.

4.2 Verification of the proposed model

The predicted interfacial bond strength and loading−displacement curves are shown in Tab.5 and Fig.9, respectively. By comparing these curves with the experimental results, the differences ranged from −1.9% to 7.1%, indicating a relatively close agreement between interfacial bond strengths predicted by the numerical model and measured by the experimental data.

Moreover, as shown in Fig.9(a)–Fig.9(d), the load−displacement behavior predicted using the finite element model agreed well with the test load displacement results. Evidently, the developed model demonstrates remarkable performance in predicting the interfacial shear bond behavior subjected to F–T cycles.

4.3 Predictions of the interfacial shear bond strength subjected to freeze–thaw cycles

To better understand the failure mechanism of the interface bond during numerous F–T cycles, interfacial specimens subjected to 50, 150, 250, and 400 F–T cycles were simulated, and the numerical results are listed in Tab.4. Evidently, the interfacial shear bond significantly decreased with an increasing number of F–T cycles. Based on the previous studies [33,34], the shear bond strength between BFRP bar and concrete without F–T cycles can be expressed as follows:

(7){\tau }_{\text{i}} =\frac{k_1\sqrt{f_{\mathrm{c}\mathrm{u}}}}{d^{k_2}},

where f_cu represents the compressive strength of concrete and k₁ and k₂ are coefficients set as 244.93 and 1.85, respectively [27].

In our study, the F–T damage factor D_F–T decreased exponentially with the number of F–T cycles and can be expressed as:

(8)D_{\mathrm{F}-\mathrm{T}}=\frac{{\tau }_{\mathrm{i}}^{\mathrm{F}-\mathrm{T}}}{{\tau }_{\text{i}}}={\mathrm{e}}^{\gamma \cdot N_{\mathrm{F}-\mathrm{T}}},

where N_F–T is the number of F–T cycles, and γ represents the experimental parameter. The relationship between the D_F–T and N_F–T is illustrated in Fig.10. The parameter γ is equal to −3 × 10⁻³, and the high correlation coefficient of 0.99 indicates that Eq. (9) can be used to estimate the interfacial bond strength for specimens that underwent F–T cycles. Notably, the value of the F–T damage factor presented in this paper cannot be applied to other BFRP bars directly. However, it can be revised based on their bond strength with the concrete matrix.

(9){\tau }_{\mathrm{i}}^{\mathrm{F}-\mathrm{T}}= {\tau }_{\text{i}} \cdot {\mathrm{e}}^{-0.003\cdot N_{\mathrm{F}-\mathrm{T}}}.

5 Conclusions

Pull-out experiments were conducted to investigate the effects of concrete strength, embedment length, and diameter of BFRP bar on the interfacial shear bond performance after subjecting the interface to numerous F–T cycles. Moreover, the interfacial specimens were simulated using the finite element method, and the simulation results were validated by experimental data. Finally, a mechanical model was developed to predict the shear bond strength of the BFRP-concrete interface after F–T cycles. The conclusions are as follow.

1) Test results showed that F–T cycles were significantly detrimental to the shear bond between the BFRP bar and concrete. Interfacial specimens subjected to 100, 200, and 300 F–T cycles showed 24.0%, 43.4%, and 52.5% lower bond strength, respectively, compared to specimens without F–T cycles. Furthermore, this study discussed the effects of F–T cycles on the degradation, deformability, and energy absorption of the interfacial specimen.

2) The shear bond strength, deformability, and energy absorption were improved by increasing the concrete strength or decreasing the embedment length and diameter of the BFRP bar if the interface was subjected to F–T cycles. However, these improvements are slower than those observed without F–T cycles.

3) The developed numerical model accurately predicted the interfacial bond strength with numerous F–T cycles. The differences between experimental and predicted results ranged from −1.9% to 7.1%.

4) The proposed mechanical model provided a numerical prediction of the shear bond for the BFRP-concrete interface subjected to different numbers of F–T cycles.

5) The current work confirmed the effects of F–T cycles on interfacial performance degradation in terms of shear bond strength, stress−slip curves, and energy absorption. Further studies are needed to confirm the behavior of BFRP-reinforced concrete structures.

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